Electrochemistry

• Electrochemistry is the study of the relationship between chemical changes and electricity.
• Redox reactions generate and use e-

  • The e- flow can be harnessed.

  • Corrosion is an electrochemical reaction.
• Applied e- flow can:

  • Drive reactant-favored reactions to products.

  • Recharge some batteries, cause electroplating…
• Oxidation & reduction always occur together.
• Reduction = gain of e- = decrease in Ox, Oxidation = loss of e- = increase in Ox
• Half-reactions may include different numbers of e-:

  
• e- must balance in the full reaction.

  
• Electrochemical Cells is the device that can generate electrical energy from the chemical reactions occurring in it, or use the electrical energy supplied to it to facilitate chemical reactions in it.
• A voltaic cell is an electrochemical cell in which a chemical reaction and electricity interact.

  Also called a battery.(A battery is strictly a series of linked voltaic cells)
• An oxidizing agent and a reducing agent are arranged so that they can react only if electrons flow through an external conductor.
• Electrodes (anode & cathode) Allow e- to pass in/out of solution.
• Salt bridge contains a salt paste (e.g. K2SO4 ).

  • A porous plug restricts bulk flow.

  • Ions pass into/out of the cells.

  • Stops charge buildup.
• In all voltaic cells:

  • The redox reaction must be product-favored.

  • Oxidation occurs at the anode.

  • Reduction occurs at the cathode.

  • Electrons move from anode to cathode through an external ‘wire’.

  • The electrical circuit is completed by movement of ions into and out of a salt bridge.
• Cell Potential & Half Cells

  Potentials (voltages), like height, can only be measured between two points:

  • A voltage can be measured between half cells.

  • An elevation can be measured between sea-level and another point.
• Overall voltage of a cell is:

  Half-cell potentials depend upon many factors, so a set of standard conditions are defined.

  
• A standard potential (E°) occurs when:

  • All [solute] = 1 M (or saturated if solubility < 1 M).

  • All gases have P = 1 bar.

  • All solids are pure.
• If E°cell is positive the reaction is product favored

  (If E°cell < 0 the reaction is reactant favored).

  E°cell = E°cathode - E°anode

  E° does not depend on nreactant or nproduct

  (i.e not on the stoichiometric coefficients).
• E°cell = E°cathode – E°anode = E°Cu2+ – E°Zn2+

  = +0.340 V – (-0.763 V)

  = +1.103 V

  
• • E° does not depend on nreactant or nproduct.
  • It is independent of stoichiometric coefficients.
  • Standard half-cell potentials do not depend on the direction in which they are written.
  • The strongest elemental oxidizing agents are in the upper right of the periodic table; the strongest reducing agents are on the left.

  
• E°cell = E°cathode – E°anode = E°Sn2+ – E°Al3+

  = -0.1375 V – (-1.676 V) = +1.539 V

  ^Not 2(-0.1375) ^Not 3(-1.676)

  
• Identify:

  • the strongest oxidizing agent. = Au^3+(aq)
  • the strongest reducing agent. = K^+(aq)
  • the weakest reducing agent. = K(s)
  • the weakest oxidizing agent. = Au(s)

  
• a.) No

  b.) Yes

  c.) Cl2 and Au^3+